The present invention relates in general to integrated circuits and, more particularly, to amplifiers in programmable analog arrays.
Programmable analog arrays are integrated circuits that include a plurality of programmable analog cells for performing analog operations such as amplifying, filtering, etc., on analog signals. The analog signals are transferred among the programmable analog cells on interconnect paths formed in routing channels between adjacent cells.
Many programmable analog functions are executed by operational amplifiers controlled by feedback circuits that include switchable passive components such as resistors and capacitors whose ratios determine the operation of the amplifiers. Each passive component is coupled in series with a transmission gate or other analog switching device controlled by data stored in a memory for coupling the component to the amplifier in a desired configuration.
When the passive components are capacitors, the amplifiers often are clocked to operate the capacitors as functional equivalents of resistors. In order to provide maximum functionality, the amplifiers are designed to operate over a clock frequency range from a few hertz up to at least four megahertz to have a high frequency capability yet be stable under low frequency conditions.
Typically, the amplifiers are configured to be fully differential in order to increase noise immunity and minimize noise coupling to other circuits. To avoid unnecessarily reducing the dynamic range of the amplifier, control must be asserted over common-mode voltage levels. Prior art amplifiers control the output common-mode voltage by developing the common-mode component of the output signal for comparing to a reference voltage to produce a common-mode correction signal. The correction signal is amplified and fed back to an adjustment node input of the amplifier to adjust the common-mode component. A disadvantage of prior art amplifiers is the extra feedback amplifier, which introduces a phase shift in the feedback path that can increase signal settling time and makes the loop difficult to stabilize and results in restrictions on the allowable range of operating conditions. The feedback amplifiers have the further disadvantage of occupying a large die area and increasing power consumption.
Hence, a circuit is needed for controlling the common-mode voltage of an amplifier which is stable over a broad range of operating conditions and operates at a high speed. It would be a benefit if the circuit reduced power consumption and die area.